Flame-failure safeguard apparatus



Oct. 26, 1954 v E. c. THOMSON FLAME-FAILURE SAFEGUARD APPARATUS 3Sheets-Sheet 3 Filed March 27, '1952 @L NQ irl Patented Oct. 26, 1954FLAME-FAILURE SAFEGUARD APPARATUS Elihu Craigy Thomson, Boston, Mass.,assignor to Electronics Corporation of America, a corporation ofMassachusetts Application March 27, 1952, Serial N0. 27 8,903

Another object of this invention is towreliablyy indicate the particularburners yof va multi-'burner heating system which have undergoneanuntimely flame failure.

Another object-of this invention is to provide multi-burner controlapparatus which reliably and continuously self-monitors its ability-toperform its prescribed safeguard functions.

Numerous heating systems in the prior art re duire combustion chambershaving a plurality of burners. It can be readily appreciated thatiffewer than all of these burnersignite during a heating period, adangerous accumulation of unburned fuel vapors might possibly collectwithin a combustion chamber so that subsequent ignition thereof by one.of the properly operating burners or by a hot refractory, fory example,results in a destructive blast. In View of the dangers inherent in theoperation of fuel burners,

it has been customary in the `p-rior art to utilize name-failuresafeguard apparatus to terminate the iiow of fuel to a supervisedcombustion chamber in response to flame failure. In the case ofcombustion chambers having a plurality of burners wherein a large dangerpotential is created by a high rate of fuel flow, it is the preferredpractice to operate the safeguard apparatus in such a manner that fuelflow to all of the burners is cut off unless all of the burners areoperating properly.

Certain multi-burner safeguard controls of the prior art have individualdetector-amplier units associated with each supervised burner so thatthe amplifier output currents generated in response to detected flameoperate a relay whose contact is connected in series with the contactsof similar relays actuated by the other detector'- amplier units. Aclosed series of relay contacts assures the continuity of the energizingcircuit for the burner main fuel valve, thereby providing for anuninterrupted flow of fuel to the combustion chamber'. if,V however, oneor more of the detector-amplifier units fails to operate its assooiatedrelay, the series contact chain is'broken and the main fuel valve isclosed, thereby terminating the flow of fuel to the. burners.

In accordance with a preferred embodiment of this invention, safeguardapparatusiofimproved reliability, which utilizes the chain mode ofoperation, is.A attained byu electronically mixing the output signalsfrom a plurality of detector-amplifier units'individuallyassociatedvvith the burners of a multi-burner assembly, rather thanhaving each amplifier terminate in a relay and placing the relaycontacts in series to complete the energizing circuit for the mainfuelvalve.

Specifically, an electronic mixing stage is provided for each of thesupervised burners. If flame is detected by a photocell which isoptically coupled to the burner, sufficient anode potential is appliedtothe electronic mixer by its associated amplierso thatan` input signalapplied to the mixer grid circuit will be transmitted to the mixeroutput. If no flame is detected by the detectoramplier-input,insufficient anode potential will be applied to the electronic mixer topermit transmission of a mixer input signal to the mixer output. Inother words, continuous signal transmission throughout the mixer stageis dependent upon the detection of flame by the detectoramplifier unitwhich supplies the anode potentials for the mixer stage.

The electronic mixers are interconnected in such a manner that theoutput of one stage is fed into -the'gridof a second stage, and so on,whereby an open ended chain is formed. An actuating signal is appliedfrom the safeguardapparatusfpower supply to one end of the chain, andthe other end of this chain applies the actuating signal, which has beentransmitted throughout the chain-connected electronic mixers,v to amaster relay circuit which controls the operation of the fuel solenoidvalve for the burners. Each electronic mixer thus provides a link in thechain, whichI link may be broken if its corresponding detector-amplifierunit fails to detect flame.

The electronic mixers herein described contemplate a conventionalcathode-follower circuit. The actuating vsignal is applied to the gridof a first f of these cathode-follower circuits and is transmittedvvirtually unchanged at the cathode thereof to the grid of the succeedingmixer stage, providingthe anode of said contact-follower circuit issupplied with a Ypositive'voltageof vat least a certain minimum-value.This vpositive voltage is generated lby the associated detector-amplierstage in response to the-detection of burner flame.

Another. feature of operation of vthis invention is a novel indicatorcircuitwhich includes an individual neonlamp interconnecting eachelectronic vmixerxand the safeguard-apparatus power supply insuch amanner that flame failure can be'k attributed to a particular burner bythe oper- 3 ation of one of said lamps. The operation of one of theselamps immediately identifies the particular burner Whose flame hasfailed so that the necessary repairs can be easily made.

Another feature of operation of this invention is the interconnection ofa second individual neon lamp between each electronic mixer and themaster relay circuit in such a manner that said lamp will illuminate ifthe supervisory apparatus is incapable of performing its prescribedfunctions. The illumination of this lamp during the standby intervalsbetween successively heating cycles shows that repairs and maintenanceare required before the apparatus can perform its functions.

In order that all of the features of this invention and the mode ofoperation thereof may be readily understood, a detailed descriptionfollows hereinafter with particular reference being made to thedrawings, wherein:

Fig. 1 is a functional block diagram showing the chain theory ofmulti-burner supervision;

Fig. 2 is a functional block diagram of a preferred embodiment of theinvention hereof which is characterized by the chain type mode ofoperation set forth in Fig. 1;

Fig. 3 is an assembly diagram showing the proper physical relationshipof Figs. 4 and 5 when viewing the schematic circuit diagram thereof; and

Figs. 4 and 5, when assembled as shown in Fig. 3, provide a completeschematic diagram of the multi-burner control described functionally inFig. 2.

Referring now to Fig. 1, a plurality of burners numbered B1 to B4 areindividually monitored by corresponding fire detector units P1 to P4.The output of each of these detector units is transmitted tocorresponding amplifier units l to The power supply furnishes theoperating potentials for the fire detectors and the amplifier units, andalso the currents which comprise the chaintransmitted actuating signal.It is contemplated that if each of the amplifier units is receiving aflame signal from its associated detector unit, the actuating signalwill be transmitted from the power supply in a step-by-step mannerthroughout all four amplifier units until it reaches the relay circuit.If the actuating signal is transmitted thusly during burner operation,fuel fiow to all of the burners is continued. If, however, one or moreof burners B1 to B4 is extinguished during a heating period, thecorresponding amplifier units open the transmission path for theactuating signal so that said signal is no longer transmitted to therelay circuit. If the actuating signal is not transmitted to the relaycircuit, a conventional fuel solenoid valve (not shown) is closed,thereby stopping the flow of fuel to all of the burners. It should beunderstood, that from a generic aspect, the structure herein describedmay be utilized to promote a variety of obvious control functions inresponse to fiame failure in one or more of a set of supervised burners.

Referring now to Fig. 2, each of the amplifiermixer units thereofcorresponds generally to the amplifier unit blocks of Fig. 1. The inputsignal to each of these amplifier-mixer units is provided by anassociated photoelectric detector unit P. This signal is applieddirectly to the input of a high-pass amplifier. The output of thehigh-pass amplifier is transmitted throughout a conventional 60 cyclenull filter to the input of a lowpass amplifier. The high-passamplifier, 60 cycle null filter, and low-pass amplier together comprisea band pass alternating-current transmission system which readilytransmits signal components within the range of approximately 5 to 25cycles per second and relatively attenuates all other signal components.

As disclosed in the copending applications of P. J. Cade and D. J.MacDougall, Serial Nos. 211,778, which is now abandoned, and 227,166,filed February 19, 1951 and May 19, 1951, respectively, improved flamesupervision can be attained by utilizing a device which responds to acertain frequency range of the alternating currents generated by theinherent amplitude nuctuations or flicker found in all types of firename. It is disclosed in the aforementioned applications that if thealternating-current components within a range of 5 to 25 cycles areutilized by a fire detection device, the possibility of erroneoussupervision is substantially eliminated because of the exclusivegeneration of frequency components within this range by fire in theusual re supervised volumes.

The output of the low-pass amplifier is transmitted to anamplifier-rectier wherein a directcurrent potential of substantialamplitude is generated during the supervision of burner fiame by thephotoelectric cell for the amplifier-mixer unit. This direct-currentpotential contributes to the energizing currents for the mixer circuitin such a way that transmission of the actuating signal AS, which isgenerated by the power supply, from the power supply and through eachmixer, depends upon the application of a similar positive potential toall of the mixer stages by their respective amplifier-rectifier units.

It can be readily appreciated that if each of burners B1 to B4 is inoperation, and that if their respective amplifier-rectier units areapplying the requisite direct-current potential to their associatedmixer circuits, the actuating signal will be transmitted from the powersupply throughout each of the mixer units to the relay circuit. If forany reason one or more of the channels should fail to generate thedirect-current potential in response to detected fire fiame,insufficient anode potential will be applied to the vacuum tube of theparticular mixer circuits to permit the actuating signal to betransmitted therethrough. When this occurs, the relay circuitdeenergizes fuel valve FV located in the main supply pipe for burners B1to B4 so that all of the burners are shut down in response to fiamefailure in one of said burners.

A flame-failure indicator neon lamp N1 .is associated with each of theamplifier-mixer units. Illumination of one of said neon lamps gives avisual indication of the particular burner in which iiame has failed.The structure which operates these flame-failure indicator lampsincludes conductor DC1 which applies a directcurrent potential from thepower supply to the multiple-connected electrodes of' each of saidlamps. The other electrodes of each of these lamps are connected to theassociated mixer circuits so that the direct-current potentialsgenerated in response to detected fire oppose the potential applied byconductor D01, thereby preventing operation of said lamps. If flamefails at one of the supervised burners, the directcurrent potential atthe corresponding mixer circuit is removed so that the potential appliedby conductor DC1 is no longer opposed. This operation results inillumination of the particular neon lamp associated with the burner inwhich flame has failed.

Conductor DC2 supplies the operating currents for the ampliiier-mixerunits.

An individual false-llame indicator lamp N2 is also associated witheachof the amplifier-mixer units, Illumination of one of these lamps indi--cates thatits associated amplifier-mixer unit is operating improperlydue to component failure, so that a flame indication is given at thecorresponding mixer stage when in fact no flame is detected.

There is shown in Figs. 4 and 5 a preferred schematic embodiment of theinvention. The mode of operation thereof corresponds to that of theblock diagram of Fig. 2. The particular multi-burner unit showndiagrammatically in Fig. 4 has an upper capped burner and two lowerburners B1 and B2. The upper burner has been capped merely to show theease with which one or more amplifier mixer units may be removed oradded as the number of supervised burners varies. In a practicalinstallation, it is contemplated that a single amplifier-mixer unit beclosely associated physically with each of the burners. The componentsof each of the amplier-mixer units are therefore preferably housedwithin individual housings, and the external connections therebetweenare preferably made by cable connections to connectors or plugin jackmeans. In view of the fact that the upper burner is capped, economy ofcomponents is obtained by providing a dummy plug with a jumper tointerconnect cable connectors V and W. This arrangement provides for thenecessary interconnection of the amplifier-mixer units which superviseoperative burners. Each of the amplifier-mixer units is identicalschematically, and therefore only the lowermost unit, which supervisesburner B2, has been shown in detail. The specific description will beconfinedV to this unit.

Photoelectric cell P2, which is preferably a photoconductive type, isoptically coupled to burner B2. This photoconductive cell iselectrically connected tothe input of its'amplifiermixer unit by meansof` conventional cable and i connector means.

A solenoid fuel valve FV is located in the main fuel supply pipe for allof the burners. Energization of this fuel valve permits the flow of fuelto the burners, whereas deenergization of the valve i cuts olf the flowof fuel to the burners. The output potential of the secondary Winding ofignition transformer IT is applied to ignition electrodes E1 and E2 sothat the burners may be ignited in response'to a heat demand. Theenergizing currents for both fuel valve FV and ignition transformer ITare supplied by secondary winding 52 through contact i2 of interlockingrelay I.

In order to simplify the representation of the connections to relays Rand I, most of the contacts thereof have been shown in removedrelationship with respect to the windings which actuate these contacts.Both normally open and normally closed contacts are identified by lowercase letters which correspond to the capital letter designations appliedto the actuating relay winding. t will be noted, for example, that relayR actuates normally-closed contact r1, which is located in theenergizing circuit for the primary winding of ignition transformer IT.

Each of the amplifier-mixer units comprises vacuum tubes Tito T4.Generally speaking, T1 and its associated components are a high-passamplifier passing frequencies above-5 cycles per second, T2 and itsassociated components are a low-pass amplifier passing frequencies below25 cycles per` second, T3 and its associated components are anamplifier-rectifier, and Tl and its associated components comprise thenovel electronic mixer. Components I2, I3, i4 and I5 comprise aconventional 60 cycle null filter which connects the output of thehigh-pass amplifier to the input of the low-pass amplier.

The direct-current potential of power-supply terminal DC2 is applied tothe anodes of tubes Ti, T2 and T3 by cable conductor C3 through cableconnector U. The potential of secondary winding is applied to tubes T1,T2, T3 and T4 by cables C2 and C5 through cable connectors X and Y,respectively.

Components I and 6 comprise a conventional resistor-capacitor L-typefilter section for further filtering the potential of terminal D02before its application to photoconductive cell P2. Photoconductive cellload resistor 2 applies the relatively smooth direct-current potentialappearing at the left terminal of resistor 6 to photoconductive cell P2.Current fluctuations developed by photoconductive cell P2 in response todetected zia-me, or for that matter to any type of incident radiation,appear across load resistor 2. The flame signal Variations developedacross resistor 2 are applied to serially-connected capacitor 3,resistor and capacitor 5 so that corresponding `fariations appear at thecontrol grid-cathode space path for tube T1. The amplified signal outputappears across load resistor 8.

The resistance from theupper terminal of capacitor to ground throughphotoconductive cell P2 is low compared to the resistance of resistor 4,and when "'ewed from the right terminal of resistor l, pcnents 5, fl and3 comprise a two section low-pass filter. Capacitor 9 couples the outputsignal developed across load resistor 8 to the input of the low-passfilter at resistor l', whereby all signal components developed byphotoconductive cell P2 having a frequency less than approximately 5cycles per second are fed back through this low-pass filter andconsequently do not appear across output load resistor 8. Therci'naining signal components appearing between ground and the junctionof capacitor 9 and gridreturn res' tor iE, with the exception of a bandat approximately G@ cycles, are transmitted substantially unattenuatedthroughout the conventional splii-'l `filter comprising components I2, I3, Iii and l5.

The unattenuated components transmitted through this nite-r appear atthe right terminal of resistor is and are applied to the controlgridcathode space path of 'tube T2 through resistor I6. The output fortube T2 appears across load resister Components IE, il', itl and. i9comprise a high-pass degenerative feed-back path which interconnects theinput and output circuits of tube T2. The values for these componentsare selected so that all frequency components transmitted to the inputof tube T2 having a frequency greater than approximately 25 cycles persecond are substantially attenuated.

The 6G cycle null filter comprising components l2, i3, it and i5 isutilized to interconnect the output of tube T1 to the input of tube T2,thereby positively attenuating the induced potentials generated in theband-pass amplifier comprising tubes T; and T2 by commercial powerdistribution systems.

The alternating-current frequency components between 5 and-25 cycles persecond-developed across load resistor 20 are coupled by capacitor 23 toshunt resistor combination 24-25, and serially-connected resistor 25 andcapacitor 21 connected across said resistor combination. Any potentialdeveloped across resistor 26 is applied directly to the controlgrid-cathode space path of amplifier-rectifier tube T3. The positivealternations of this potential, measured from the left to the rightterminals of resistor 25, cause tube T3 to draw considerable anodecurrent, whereas the negative alternations tend to cut off anode currentflow in tube T3. Because of this action, capacitor 2l assumes agradually rising positive direct-current potential above ground and theentire alternating-current potential applied through capacitor 23appears across resistor 2E. Equilibrium is reached across capacitor 2lwhen that portion of the direct-current potential thereof which appearsacross resistor 25 plus the peak Value of the positive alternations fromleft to right across resistor 25 exceeds slightly cut-off bias for tubeT3. When equilibrium is reached, the direct-current potential acrosscapacitor 21 is greater than that across resistor 26 by the ratio of theresistance of resistor 26 to the resistance sum of resistor 25 and theparallel combination of resistor 2li and 25. Since the direct-currentpotential across resistor 25 follows and is nearly equal and opposite tothe positive peaks of the alternating-current potential applied throughcapacitor 23, tube T3 acts as an amplifier as well as a rectifier. Thedirect-current potential appearing across capacitor 2'.' is applieddirectly to the anode of mixer tube T4 and constitutes the anodepotential for said tube.

Tube T4 is connected as a conventional cathode follower with the inputsignal thereto being applied through grid resistor 35, and the outputsignal thereof being developed across cathode load resistor 32. Theactuating signal hereinbefore described in connection with thefunctional block diagram of Fig. 2 is applied to the right terminal ofresistor 3ft in a manner which will be described in detail hereinafter.Likewise, resistors 28 and 25, and flame-failure neon lamp N1 comprise aportion of the name-failure indicator network described in connectionwith the blocl; diagram of Fig. 2. Resistor 33 and neon lamp N2 comprisea portion of the false-flame indicator network described in conjunctionwith the block diagram of Fig. 2. The detailed operation offlame-failure indicator lamp N1 and false-:dame indicator lamp N2 willbe described in detail hereinafter.

The output signal developed across cathode load resistor 32 is appliedto the control gridcathode space path of the left triode section of tubeT5 by cable conductor C4 and coupling capacitor 3l. When a signal isdeveloped across grid resistor 38, a corresponding out-of-p-hase signalis developed across load resistor 43 for the left triode section of tubeT5. The signal developed across load resistor 43 is coupled by capacitorl5 to the control grid-cathode space path of the right triode section oftube T5. If an appreciable potential is developed across resistor 41 andthereby applied to the control grid of the right triode section of tubeT5 through grid current limiting resistor 45, relay R is operated.Secondary winding 52 supplies the negative bias for the right triodesection of tube T5, and resistor 39 supplies the bias for the lefttriode section of tube T5.

Secondary winding 5U supplies the alternating plate current for theright triode section of tube T5, Capacitor 44 shunts the winding ofrelay R so 8 that the characteristic alternating-current chatter iseliminated. It should be noted that the bias potential applied to theright triode section of tube T5 by secondary winding 62 is in such adirection as to oppose current flow in the right triode section of tubeT5 due to the potential applied thereto by secondary winding 50. Thedotted terminals of both secondary windings 55 and 62 indicate identicalpolarities.

The anodes of full-wave rectifier tube T7 are connected to the endterminals of serially-connected windings 6U and 6l. The filament of tubeTv is connected directly to secondary winding 59.

Conventional full-wave pulsating direct-current appears at the cathodeof tube T7 and is applied to the input of the resistor-capacitor filtersection comprising components 5I, 52 and 53. The ltered direct-currentoutput appears at terminal DCz.

serially-connected resistors 51 and 58 provide a voltage divider for thesecondary potential of winding 6l. The junction of resistors 5l and 58is connected to terminal DCz by means of seriallyconnected resistors 55and 56. The values for resistors 55, 55, 5l and 58 are so selected thatthe potential of terminal AS with respect to ground is a direct-currentpotential with a smaller Valued alternating-current potentialsuperimposed thereupon. This composite potential comprises the actuatingsignal for the -chain-connected electronic mixers hereinbeforedescribed.

This actuating signal is applied to cable connector V of the dummy plugby means of cable C7. The jumper within the plug transmits the actuatingsignal to cable connector W, at which point the signal is transmitted tocable connector V of the amplifier-mixer unit for burner B1. This signalis further transmitted to cable connector W for the amplifier-mixer ofburner B1 by a mixer identical to that of tube T4, providing anodepotential is supplied to the mixer tube, whereupon the signal is stillfurther transmitted to cable connector V for the amplifier-mixer unitwhich supervises burner B2. This signal is then applied to the controlgrid of mixer tube T4 through grid resistor 34. If suicient potential isapplied to the anode of mixer tube T4 by capacitor 2l in response to thedetection of fire flame by photoconductive cell P2, a correspondingoutput signal will be developed across resistor 32, which signal isapplied to the control grid of the left triode section of tube T5 bymeans of cable conductor C4 and coupling capacitor 31.

The complete transmission of the actuating signal throughout the dummyplug and the mixer circuits for both of the amplifier-mixer units backto the control grid of tube T5 develops an output signal across loadresistor i3 which causes the right triode section of tube T5 to operaterelay R. The closure of the contact r1 actuated by relay R assures thatan operating potential is constantly applied to fuel valve FV wherebyburner fuel flow is maintained. If, however, relay R should release itscontacts in response to the incomplete transmission of the actuatingsignal as hereinbefore described, the energizing circuit for fuel ValveFV will be opened and consequent closure of the fuel valve willterminate the flow of fuel to burners B1 and B2. This operation isprovided by the specific actions of the circuitry which includesthermostat TH, manualstarting push button PB and its shunt contact r3,and parallel-connected contacts r2 and is. Relay I is an interlock relaywhich prevents the startingl up of the burners when one or more of theV...amplifier units `is incapable `of performing its prescribedsafeguard functions.

ing resistors 28 and 29. The plate potential applied to mixer tube T4 isalsoapplied to seriallyconnected resistors 28 and 29 so that ,theportion thereof across resistor 29 appears vat one of the electrodes forneon lamp N1. If flame is detected by photoconductive cell P2 with theresulting anode potential for tube T4 appearing across resistors 2t and29, neon lamp N1 will not illuminate because the potentialacrossresistor 29 opposes the potential appearing at terminal DC1. If,however, photcconductive cell P2 fails to detect flame so that adirect-current potential no longer appears across resistors 28 and 29,the potential applied to neon lamp N1 by terminal DC1 is sufcientlylarge to illuminate neon lamp N1. It is therefore seen that theillumination of neon lamp N1 is indicative of a flame-failure conditionat its associated burner. The breakdown of one of said neon lamps N1increases .the voltage drop across resistor 49 whereby the supp-lyvoltage at DC is too low to permit the remaining lamps N1 to fire.

The anode of mixer tube T4 is connected to the control grid of the righttriode section of tube T by serially-connected resistor 33, neon lampN2, cable conductor C1, normally closed contact i1, and resistor 45.This particular interconnection provides for'the illumination of neonlamp N2 in response to a false-'flame condition in its associatedamplifier-mixer unit. Inparticular, if component failure within theamplifiermiXer unit causes a potential to appear at the anode of Amixertube T4 when no flame is being supervised by photoconductive cell P2,thispositive potential breaks down neon lamp N2 causing the right triodesection of tube T5 to actuate relay R, by supplying a positivedirect-current potential through contact i1 to resistor 4l. Theoperation of relay R locks out the safeguard control circuit by openingnormally-closed contact r2, so that fuel valve FV cannot be closed inresponse to a heat demand until the defective components which createdthe false-flame condition have been repaired or eliminated.

The detailed sequence of operation of the schematic circuit of Figs. 4and 5 is as follows. An alternating-current line potential iscontinuously applied to primary winding 64 of power transformer 63during both heating and standby periods.

During the standby periods, the direct-current potential at terminal D02is applied to tubes T1, T2, Ts and the right triode section of tube T5.The alternating-current potential of secondary winding @d is alsoapplied to the cathode-anode space path of the right triode section oftube T5 through the winding of relay R. In view of the fact thatphotoconductive cells P1 and P2 are not observing flame, the actuatingsignal appearing at terminal AS is not transmitted throughout theelectronic mixers. As a consequence, the

T10 biasing potential applied by secondary winding E2 to 'the righttriode section of tube Tt is suinciently great to prevent the operationof relay R. Interlock relay I is also unoperated due to the fact thatthermostat TH is open.

The flame-failure indicator lamp N1 associated with the burner which wasirst extinguished at the termination of the prior heating period remainscontinuously illuminated throughout the Succeeding standby period. Thisis because the prior removal of the potential across the correspondingresistor 39 no longer opposes the breakdown potential appearing atterminal DC1, and all of the other flame-failure neon lamps which aremultiple-connected to the operated neon lamp N1 are locked out in theconventional manner characteristic of parallel-connected negativeimpedance devices. Therefore only the naine-failure indicator lamp ofthe burne11 which was first extinguished at the termination of the priorheating period remains illuminated during a succeeding standby period.

Bimetallic thermostat TH closes its contacts in response to a heatdemand at burners B1 and Bz. Thereafter, push button PB is manuallyclosed providing for the operation of interlock relay I by current iiowfrom secondary winding 52 in a path 'which comprises thermostat pushbutton normally-closed contact rz, and the winding of interlock relay Iback to secondary 52. The operation of interlock relay I closes contactthereby applying the potential of secondary winding 62 to fuel valve FVand the primary winding of ignition transformer IT. Fuel valve FV istherefore opened providing for the flow of fuel to burners B1 and B2. Inview of the fact that relay R is released at this time, the potential ofsecondary winding 62 is also applied to the primary of ignitiontransformer IT through normally-closed contact r1. The high voltageignition potential appearing at electrodes E1 and E2 ignite the fuelyappearing at burners B1 B2 in the conventional manner.

The detection of the burner flames by associated photoconductive cellsP1 and P2 causes a direct-current potential to be applied to the vnodesof their respective mixer tubes in a inanner hereinbefore described.With the application of the plate potentials to the mixer tubes, theactuating signal appearing at terminal AS is transm' ted throughout thedummy plug, the first ampliier-mixer unit, and the second ampliner-mixerunit back to the control grid of the left triode section of tube T5. Thecorresponding signal developed across load resistor applies a potentialto the cintrol grid of the rig t triode section of tube T5 so that thebias potential of secondary winding 62 is overcome and relay R isenergized. The operation of relay R opens normally-closed contact r1thereby terminating the ignition potential at burners B1 and B2.

After all of the burners have started-up and relay R closes contact rsthereby shorting push button PE1 the push button can be manuallyreleased.

If flame should fail during a heating period at burner B2, for example,the operating potential at the anode of mixer tube T4 would disappear,and the actuating signal applied to the control grid of mixer tube Tiwould no longer appear across cathode load resistor 32. Consequently,relay R would release its contacts, thereby opening the energizingcircuit for relay I. The opening of contact i2 would deenergize fuelvalve FV .and shut down burners B1 and B2. Flame-failure indicator lampN1 associated with burner B2 would also be ignited, because thepotential developed across the corresponding resistor` 29 would nolonger oppose the breakdown potential appearing at terminal DC1. Allother llamefailure indicator lamps would be locked out.

If prior to the starting up of burners B1 and Bz in response to heatdemand, component failure within one f the amplifier-mixer units shouldcause the simulation of a flame condition when in fact no ame existed inany of the burners, the potential falsely appearing at the anode of themixer stage would be transmitted to the control grid of the right triodesection of tube T by the corresponding resistor 33, false-flameindicator lamp N2, cable conductor C1 and normally closed Contact i1. Asa consequence, lamp N2 Would become illuminated thereby giving a visualindication of an improper operating amplifier-mixer unit. The lowsustaining potential across the electrodes of lamp N2 after breakdownwould raise the potential of the control grid of the right triodesection of tube T5 a sufficient amount above ground to permit anodecurrent flow in that triode section to operate relay R. The operation ofrelay R during the standby period would open normally-closed contact T2so that subsequent closure of thermostat TH in response to a heatdemand, together with manual operation of push button PB, would fail tocomplete the energizing circuit for interlock relay I. This mode ofoperation assures that fuel will not be supplied to burners B1 and B2unless the safeguard apparatus is fully capable of performing itsprescribed functions.

It is to be understood that the above-described arrangements areillustrative of the applications of the principles of this invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the scope of the invention.

What is claimed is:

l. In apparatus for supervising a burner flame, a source of activatingsignal, a relay to be operated by said activating signal, and a controlcircuit to apply said activating signal to said relay, said controlcircuit comprising a cathode follower having an input electrode and anoutput electrode, means to apply said activating signal to said inputelectrode, means connecting said output electrode to said relay,potential supply means to render said cathode follower operative, amedetecting means, and means to connect electrically said potential supplymeans to said cathode follower only when a flame is detected by saiddetecting means.

2. In apparatus for supervising a burner flame, a source of activatingsignal, a relay to be operated by said activating signal, and a, controlcircuit to apply said activating signal to said relay, said controlcircuit comprising a cathode follower having a cathode, a grid and ananode, means to apply said activating signal to said grid, meansconnecting said cathode to said relay, anode potential supply means torender said cathode follower operative, iiame detecting means, and meansto connect electrically said anode potential supply means to said anodeonly when a flame is detected by said detecting means.

3. In apparatus for supervising a plurality of burners, a source ofactivating signal, a relay to be operated by said activating signal, anda control circuit to apply said activating signal to said relay, saidcontrol circuit comprising a plurality of series-connected cathodefollowers, each having an input electrode and an output electrode, meansto apply said activating signal to the input electrode of the first ofsaid cathode followers, means connecting the output electrode of thelast of said cathode followers to said relay, potential supply meansassociated with each cathode follower to render it operative, flamedetecting means associated with each potential supply means, and meansto connect electrically said potential supply means to its associatedcathode follower only when a flame is detected by said detecting means.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,212,352 Plein Aug. 20, 1940 2,282,551 Yates May 12, 19422,410,524 Richardson et al. Nov. 5, 1946 2,416,781 Thomson Mar. 4, 19472,431,158 Yates Nov. 18, 1947 2,567,036 Shannon Sept. 4, 1951

